Apparatus for and method of sensing particulate matter

ABSTRACT

This invention concerns sensing particulate matter entrained in a gas flow that is isokinetically sampled. The sample is flowed past an ungrounded electrode with which the particles in the sample impact and transfer electrical charge. The sample also flows past a grounded electrode to complete a circuit. Measurement is made of the minute current flow in the circuit, which current is linear with respect to the square of the average velocity of the particles impelled by the gas flow.

United States Patent [191 Smith [451 Feb. 26, 1974 APPARATUS FOR ANDMETHOD OF SENSING PARTICULATE MATTER Thomas B. Smith, Brighton, Mass.

Ikor Incorporated, Burlington, Mass.

Filed: Dec. 8, 1971 Appl. No.: 205,784

Inventor:

Assignee:

US. Cl. 324/32, 324/71 R, 73/194 F,

73/28 Int. Cl G01r 5/28- Field of Search ..324/32, 33, 71 CP; 73/28,

73/194 F, 421.5 R, 421.5 A, 422

References Cited UNITED STATES PATENTS 12/1967 Dimick 324/32 6/1966 Rich..324/32 OTHER PUBLICATIONS R. L. Solnick-Sampling ParticulateMatter-The Oil and Gas Joumal-lO/l5/56, pp. 120-424.

M. S. Beck-Electrostatic Charge Measurement of Particulate MaterialsBeing Transported at High Velocity-Proceedings of the Conference onDielectric Materials Measurements and Applications-July, 1970, pp.38-41.

Primary ExaminerMichael J. Lynch Attorney, Agent, or FirmSchiller &Pandiscio [57] ABSTRACT 3 Claims, 4 Drawing Figures I 29 o 64 I 1 i 5OCONTROL I I 22 5 60 28 as a I I 58 52 4(5 PRE 1 5. 1 CONDITIONER a.

This invention relates to the sensing of particles in a fluid stream,and more particularly to monitoring particulate matter contained in gasstreams,'as in the stack discharge from incinerators, electric utilitygenerators and the like.

In many cases, air pollution is not only created by the discharge ofgases per se into the atmosphere but also by particulate matter in thegas stream. While efforts have been made to control particle emission,as by filtering and precipitation techniques, it has been difficult toascertain accurately the effectiveness of such antipollution controlequipment.

In order to determine the extent of pollution by particle emission, orto determine how effective antiprovide a system capable of monitoringparticle emission in a gas stream on a substantially continuous basis.

Monitors have long been known which collect or sample particulate matterfrom a mainstream, usually in a filter so that the collected matter canlater be weighed. lf the-sampling technique provides samples which areproportional to the concentration of matter in the main stream, one canthereby determine that concentration from the weight of the collectedsample and the volume of gas passed through the filter. U.S. Pat. No.1,100,171 typifies such prior art, but unfortunately is a batch processwhich does not lend itself to continuous monitoring, nor is it sensitiveto very small amounts of particulates in large volumes of gas.

One can detect the presence of particulate matter in the gas flow byoptical techniques which are highly dependent upon the size of theparticles, the reflectivity of the particles and the speed with whichthe particles are moved by the gas stream past the detector, allparameters having very wide ranges. Consequently, optical methods arequite limited in the scope of their application..

It is well known that one can detect the presence of particulate matterin the gas flow by passing the gas between a pair of electrodes acrosswhich an electrical potential is maintained, ordinarily at a levelsomewhat lower than will sustain a corona discharge for the dielectricproperties of that gas. This technique depends upon the change indielectric properties between the electrodes due to the introduction ofparticulate matter, because changes in the dielectric may cause a largcurrent to flow between the electrodes.

Such electrical techniques of course, require substantial voltagesources, are hazardous where the gases and- /or particles are explosive,and have been found to have limited sensitivity in the detection of verysmall quantities of particulates in a gas stream. I

Regardless of the detection method used, in order to obtain a trulyrepresentative sample of a gas flow, the sample should be takenisokinetically, i.e., introduced into the sampling system withoutsubstantial perturbation of the kinetics of the gas being sampled. vAnumber of isokinetic sampling systems have been proposed but are notbelieved to provide sampling of sufficient precision when very smallquantities of particulates in very large amounts of gas are involved.

A principal object of the present invention is to provide a method ofand apparatus for accurately determining the flow rate and for theconcentration of parpollution control equipment may be, it is desirableto ticulate matter in a gas stream. Yet another object of the presentinvention is to provide such method and apparatus wherein isokineticsampling can be achieved with a high degree of precision. Yet anotherobject of the present invention is to provide an improved andsimplified, yet highly sensitive, detector for the presence ofparticulate matter in a gas flow. Another object of the presentinvention is to provide a system so highly sensitive that it can beemployed as a high volume sampler for monitoring ambient air quality.

The foregoing and other objects of the present invention are generallyachieved by providing a sampling tube having an orifice disposedsubstantially normally to the flow of a main gas stream being sampled, aPitot tube arrangement disposed adjacent the sampling orifice formeasuring a pressure differential indicative of the velocity of the maingas stream, and a Venturi type arrangement within the tube adjacent theorifice for measuring a pressure differential indicative of the velocityof a sampled gas stream in the tube, both pressure-measuringarrangements being exposed thereby to substantially the same ambienttemperature which is also measured. The particulates are detected byproviding at least a pair of normally uncharged, spaced electrodesdisposed to permit flow therebetween of at least a part of the sampledgas stream containing a correspondingly proportional amount ofparticulate matter therein. Means are provided for measuring across theelectrodes minute electrical current which is due to the triboelectricproperties of particles moving in a gas stream, and for measuring thetemperature of the gas stream adjacent the electrodes. From the datathus obtained, one can determine the weight of particulates per unittime flowing in the gas stream and also the concentration ofparticulates.

Other objects of the present invention will in part be obvious and willin part appear hereinafter. The invention accordingly comprises themethod comprising the several steps and the relation and order thereof,and the apparatus possessing the features, properties and relation ofelements, all of which are exemplified in the following detaileddisclosure and the scope of the application of which will be indicatedin the claims.

For a fuller understanding of the nature and objects of the presentinvention, reference should be had to the following detailed descriptiontaken in connection with the accompanying drawing wherein:

FIG. 1 is a diagram, partly in block, partly broken away, and partly infragment, showing the combination of elements embodying the principlesof the present invention;

FIG. 2 is a graphical representation showing a number of curves, eachfor a different temperature of stack velocity in terms of Ap vs. stackvelocity in feet/minute;

FIG. 3 is a graphical representation showing a number of curves, eachfor a different temperature of the relationship of stack velocity infeet/minute vs. flow rate in standard cubic feet/minute through a sensorof the invention; and

FIG. 4 is a graphical representation showing a number of curves, eachfor a different standard mass flow rate, of the relationship of massflow rate in grains/min. vs. mass loading in grains/standard cubic foot.

Referring now to FIG. 1 there is shown means for sampling a gas streamwhich, for example, flows in a stack exemplified by a fragment 20 andhaving a wall 22 extending substantially parallel to the direction ofthe flow of gas (indicated by the arrows) containing entrainedparticulate matter. The sampling means comprises a hollow, tubular probe24 having an orifice 26 at one end thereof, the orifice preferablybeingdisposed across or normal to the direction of the gas flow so thatthe gas impinging on the orifice will enter the hollow interior ofsampling tube 24. In the preferred form, the probe is formed ofstainless steel so as to reduce the corrosive effects of the stack gas.To insure a large gas flow, the inside diameter of the sampling nozzleof probe 24 is preferably around 0.9 inches except at the location of aVenturi-type constriction to be described hereinafter.

Positioned adjacent orifice 26 but downstream therefrom so as to avoidperturbing the kinetics of the gas entering orifice 26, is an S-typePitot tube 28 having the usual pair of arms or openings, one positionedto face into the main gas stream, the other positioned to face away fromthe main gas stream. The arms of Pitot tube 28 extend substantiallyparallel to tube 24 through wall 22 to the exterior of stack 20 and areconnected to a device such as meter 29 which determines and displays thepressure differential AP, across two arms of the Pitot tube. For a giventemperature, it is known that AP, is proportional tothe square of thevelocity of the gas flow. Positioned within tube 24, downstream fromorifice 26 is a Venturi arrangement 30 within tube 24, the arrangementcomprising a restricted portion or throat 32 having the usualappropriately designed tapers at inlet and outlet to minimizeturbulence, and a pair of openings, one into throat 32 at 34, the other36 being in tube 24 just upstream from throat 32. Both openings 34 and36 are attached to appropriate tubing 38 which in turn is connected to asecond device such as meter 40 for detecting the differential pressureAP, across openings 34 and 36 and for displaying same. Venturiarrangement 30 is preferably close enough to orifice 26, so that thetemperature of gas forced through throat 32 is substantially at the sameambient temperature as the gas entering orifice 26 and the gas flowingpast Pitot 28. Again, it is known that AP V where V is the velocity ofthe gas flow through the venturi.

Tube 24 is of sufficient length so as to extend through wall 22 suchthat orifice 26 can be positioned substantially centrally in stack 20,i.e., at a position of substantial gas flow with preferably a minimum ofturbulence due to wall effects. At the opposite end of tube 24 there ispositioned a pump, shown schematically at 42. Pump 42 preferably is ahigh capacity, variable speed motor/- blower which is capable of drawinggas through tube 24 from orifice 26 over a very wide range of gasvelocities, typically, for example, from one to several thousand feetper minute. Positioned adjacent the inlet to pump 42 from tube 24 is afiler 44, preferably a high efficiency, glass fiber filter, capable ofretaining all particles, for example, above 0.3 microns. Such a filteris preferably removable so that the weight difference of the filterafter a specific pumping interval or after passage of a known volume ofgas therethrough, can be readily determined. Filter 44 is howeveroptional in the device shown, although it is useful to calibrate the device.

In order to electrically sense the particulate matter traversing tube 24in the sampled gas stream, the invention includes novel sensing device46. The latter, in the form shown, constitutes a pair of electrodes, oneof which is typically formed as a cylindrical, smoothsurfaced metallicelement or bullet 48, for example 2 to 3 inches long and one-half inchin diameter, positioned coaxially within tube 24 and annularly spacedtypically about one-fourth inch from the latter. A supporting lead orrod 50 is provided extending from bullet 48 substantiallyperpendicularly to the axis of tube 24 and outwardly of the latterthrough opening 52 in a side wall thereof. Rod 50 and bullet 48 arepreferably formed of an electrically conductive material such asstainless steel or the like. In order to support rod 50 and bullet 48 intheir operative positions, the device includes tube 54 which is fittedinto tube 24 at opening 52 and sealed therein at their common joiningedges. Rod 50 is suspended coaxially within tube 54 and held in thatposition by a mass of insulating material 56. The latter is preferablyshaped substantially as a cone having its apex directed toward bullet 49and its base sealed around the edge thereof to tube 50. A plurality ofbleed holes 58 are provided, preferably disposed symmetrically aroundtube 50 so as to permit the interspace therein between tube 54 andconical surface of insulator 56, to communicate pneumatically with theair on the outside of the system. In order to avoid the entrainment ofany particulate matter into air pulled in through bleed holes 58 by theaction of pump 42, the bleed holes are preferably surrounded by anannulus 60 of filter material.

The other electrode of the sensor is typically formed of the interiorwall of an electrically conductive portion of tube 24 itself. As shown,the wall of tube 24 is connected, as by lead 62 to ground. Rod 50 isconnected to a currentmeasuring instrument such as meter 64, the circuitthrough meter 64 being completed to ground.

In a preferred form of the invention, there is included, between venturi30 and sensor 46, means, as shown schematically as preconditioner, shownas block 66, for heating the gas traversing tube 24, well after it haspassed through venturi 30, to a temperature above the dew point, therebyeliminating any problems which might occur by the condensation ofmoisture into droplets within the sensor which droplets might be sensedas particles. Thermocouples 74 and 76 are respectively located forsensing the temperature of the gas entering orifice 26 and thetemperature of the gas passing through sensor 46. Thermocouples arerespectively connected to meters 78 and 80 for displaying the sensedtemperatures.

In operation, probe tube 24 may be either permanently emplaced withinstack 20 or may be temporarily projected through an opening in wall 22,or may actually be used in free air as an air quality sampler, accordingto the needs of the operator. Tube 24 is positioned so that orifice 26faces directly into the flow stream of the gases traversing stack 20.Pump 42 is turned on to provide temporarily fixed pumping volume and thesystem allowed to equilibrate. When the differential pressure reading onmeters 29 and 40 have settled, the volume being pumped by pump 42through tube 24 can now be changed until the differential pressureindicated on meter 40 matches that displayed on meter 29. At that point,the sample of gas traversing sensor 46 is isokinetic and no furtheradjustments need be made, as for any temperature variations.

Gas passing through tube 24 and around bullet 48 and containingparticulate matter will create a current flow of magnitude typically. inthe nanoampere range and below, to occur between the electrodes, andsuch current flow is detected on meter 64. The latter must have therequisite sensitivity to such very low currents and hence usuallyincludes an input amplifier with a leakage current preferably less than0.01 picoamperes. The flow of current is, unlike the logarithmicdependence of current in devices of the type employing a high potentialimposed between the electrodes, linearly dependent upon the mass flowrate of the particulate matter between the electrodes.

It is quite important that particulate matter does not collect on eitherbullet 48 or around the edge of opening 52, because such collection willnot only unduly impede the flow of sample gas through tube 24 but if theparticulate matter collects in sufficient quantity, it may short circuitthe electrodes. To this end, bullet 48 is smooth-surfaced. Further, thepresence of opening 52 would ordinarily serve to introduce eddys andtherefore stagnant areas which would tend to favor particle deposition.However, it will be seen that the provision of the bleed holes 58insures that a substantial volume of air will be drawn through opening52 to mingle with the sampled stack gases and keep them in sufficientmotion to minimize the deposition of particulates until the latter aretrapped in filter 44.

The sensor apparently operates on the basis of charge transfer betweenthe particles in the gas and the electrodes. While the exact mechanismis not fully understood, it has been observed that the current appearsto be due to an interaction of the flowing particles with theelectrodes, apparently the impingement of a proportionate number ofparticles on an un-grounded electrode. lt is believed that the currentflow is specifically due to the pick-up of electrons from the centerelectrode by most varieties of particles, although it has also beenobserved that some types of particulate matter appear instead to donateelectrons, causing a current flow in the opposite direction. While areturn path for current flow is provided by the grounded electrode, itis not necessary that any particular spacing be provided between theelectrodes, at least for that purpose. Indeed, one can insulate theinterior wall of tube 24 adjacent bullet 48, in which case the metallicwall of tube 24 further downstream or at pump 42 will serve as theelectrode connected to system ground, without substantially altering themagnitude of the current flow.

It is also believed that the rate of charge transfer is proportional tothe total surface area of the particles flowing past the ungroundedelectrode and is also related to the triboelectric properties of theseparticles. Because for a reasonably quasi-static distribution ofparticles of known material and sizes, the total particle surface areacan be related to mass, the resulting current flow can be empiricallyrelated to the mass. The current flow ranges typically from thenanoampere magnitude (about X 10" amps) to a fraction of picoamperes(about 0.1 X 10" amperes), depending on the mass flow rate of particlesthrough the sensor and the sensor dimensions. The very small currentobserved is substantially insensitive to the temperature of the sensor.Gas velocity however is temperature variable and hence will causecurrent variations. The current is substantially linear with respect tothe square of the velocity of the particles and is also substantiallylinear with respect to the total number or mass of particles. The sensorappears quite capable of detecting particles in the submicron range, forit exhibits a response to suspensions of particles known to-be in thesubmicron range such as cigarette smoke in a quite low concentration.Ambient mass loading levels of about 1 X 10' grains/SCF yield typicallycurrents around magnitude of 0.1 picoampers for typical stack gasvelocities e.g., from 1,000 to 10,000 feet/min.

As noted, the relation between current and mass, particularly mass perunit volume, is empirical. To obtain the relationship from the raw dataof stack gas differential pressure observed at meter 29 and temperatureobserved at thermocouple 74, one first turns to the empirical chartshown at FIG. 2, selects an oridinate according to the value of AP, readon meter 29, and then proceeds along that ordinate to the curveidentified by the temperature read on meter 78. The value of theabscissa taken at the intersection of the selected ordinate andtemperature curve of FIG. 2 is then the stack gas velocity indistance/unit time.

With that value of stack gas velocity in distance/unit time as theordinate selected in FIG. 3, one proceeds along the ordinate to theappropriate curve identified by the temperature of the stack gas asdetermined by thermocouple 74 and displayed on meter 78. The value ofthe abscissa taken at the intersection of the selected ordinate andtemperature curve of FIG. 3 is then the value of the sensor flow rate,preferably in standard cubic feet per minute. It is understood that theuse of the charts presupposes that all readings have been achieved aspreviouslydescribed at equilibrium, and particularly, that AP has beenmade equal to AP, Of course, the curves of FIG. 3 are valid only for aparticular configuration and size of tube 24 and sensor 46 and needs beredetermined for each new model.

It will be readily apparent that the chart of FIG. 2 permits one toconvert a pressure differential into a temperature-corrected flowvelocity, and the chart of FIG. 3 permits conversion of that flowvelocity of the stack gas into a temperature-corrected volumetric flowrate through the sensor.

Now one can compute the mass flow rate of particulate matter (e.g.,grains/min.) through the sensor using the following empiricalrelationship:

Grains/min (ER/C 'where R is the percentage of fullscale reading ofmeter means 64;

E is a constant based upon certain physical characteristics of theparticulate material passing through the sensor;

G is the system gain usually determined by an amplifier forming part ofmeter 64;

V is the temperature-corrected flow velocity of gas through the Venturias determined from a chart such as FIG. 2; and

B is a temperature correction factor defined as:

where Tst is the stack gas absolute temperature and Tse is the sensorgas absolute temperature.

As noted the response of sensor 46 varies according to a sensitivityconstant E, and this latter is believed to be based on the triboelectricproperties of the particulate matter in the sample stream. It has beenfound that, based upon some arbitrary sensitivity constant such as forexample, a value for alumina of 1.3, other sensitivity constants canreadily be determined. Thus, on such basis, graphite is about 2.7, fueloil is about 2.2, rock dust is about 1.1 and so on. Using suchsensitivity constants, one can set the scale of meter 64 and cancalibrate it readily in a linear manner against the mass flow rate of aspecific type of particulate matter contained by the sampled gases.

To determine finally the mass loading of the gas passing through thesensor, turning to the chart of FIG. 4, one selects an ordinatedetermined by computation of mass flow rate as noted above, and proceedsalong that ordinate until arriving at the curve identified according tothe value of the sensor flow rate taken from the chart of FIG. 3. Thevalue of the abscissa where the computed ordinate and selected sensorflow rate curve intersect, is the mass loading of the sample and thus ofthe stack gas, typially in grains/standard cubic foot.

It will be apparent to those skilled in the art that the computationsabove described can be reduced for a given system to nomographs orreadily performed in ananalog or appropriately programmed digitalelectronic computer.

It will also be apparent to those skilled in the art that, although theoperation of the system has been described in connection with a manualcontrol and visual observation of meters 29 and 40, electronic controlmeans shown schematically as block 68 can also be provided forcontrolling the speed of pumping of pump 42. Thus means 68 is shownconnected by dotted lines 70 and 72 to meters 29 and 40 forautomatically controlling pump 42 so as to servo the pressure detectedat meter 40 to that detected at meter 29. The signals provided to thevarious meters can be readily digitized in analog-to-digital convertersand the entire operation controlled and computed by an appropriatelyprogrammed computer.

Since certain changes may be made in the above apparatus withoutdeparting from the scope of the invention herein involved, it isintended that all matter contained in the above description or shown inthe accompanying drawing shall be interpreted in an illustrative and notin a limiting sense.

What is claimed is: '1. Method of sensing particulate matter in a gasflow and comprising the steps of:

isokinetically sampling a portion of said gas flow; flowing said portionbetween at least two spacedapart electrodes, one of said electrodesbeing grounded; determining the volumetric rate of flow of said portionbetween said electrodes; measuring that current flow occuring betweensaid electrodes which is due to the transfer of charge between the otherof said electrodes and particulate matter moving in said portion, saidelectrodes having substantially no other potential impressedtherebetween, and; determining, as a function of said current flow andof said volumetric rate of flow, the mass flow rate of said particulatematter between said electrodes. 2. Method as defined in claim 1 whereinsaid mass flow rate (R is determined substantially in accordance withthe relationship (ER/CV2) where E is a constant based uponcharacteristics'of the particulate matter passing between saidelectrodes, R is a factor dependent linearly upon the magnitude of saidcurrent flow, G is a multiplication factor. V is the flow velocity ofsaid gas flow, and

B is a temperature correction factor based upon the absolutetemperatures of said flow and said portion. 3. Method as defined inclaim 1 including the step of determining the mass loading ofparticulate matter per unit volume of said portion of gas, as a functionof said mass flow rate and said volumetric rate.

1. Method of sensing particulate matter in a gas flow and comprising thesteps of: isokinetically sampling a portion of said gas flow; flowingsaid portion between at least two spaced-apart electrodes, one of saidelectrodes being grounded; determining the volumetric rate of flow ofsaid portion between said electrodes; measuring that current flowoccuring between said electrodes which is due to the transfer of chargebetween the other of said electrodes and particulate matter moving insaid portion, said electrodes having substantially no other potentialimpressed therebetween, and; determining, as a function of said currentflow and of said volumetric rate of flow, the mass flow rate of saidparticulate matter between said electrodes.
 2. Method as defined inclaim 1 wherein said mass flow rate (Rmf) is determined substantially inaccordance with the relationship Rmf (ER/GV2) Beta where E is a constantbased upon characteristics of the particulate matter passing betweensaid electrodes, R is a factor dependent linearly upon the magnitude ofsaid current flow, G is a multiplication factor. V is the flow velocityof said gas flow, and Beta is a temperature correction factor based uponthe absolute temperatures of said flow and said portion.
 3. Method asdefined in claim 1 including the step of determining the mass loading ofparticulate matter per unit volume of said portion of gas, as a functionof said mass flow rate and said volumetric rate.